Esterified alkoxylated glycerin and other esterified alkoxylated polyols have recently been identified as useful reduced calorie fat substitutes. Compounds of this type, which are described more fully in U.S. Pat. Nos. 4,861,613 and 4,983,329, are substantially resistant to hydrolysis upon digestion owing to the high proportion of linkages in which the carbons adjacent to oxygen in the fatty acid ester groups are secondary or tertiary in structure. In a preferred embodiment of such substances, the structure may be represented as follows: ##STR1## wherein G is a glyceryl radical, n is from about 1 to 3 on average, and R is a long chain paraffinic or olefinic hydrocarbon radical derived from a fatty acid.
However, the ability to use esterified alkoxylated polyols of this type at relatively high concentrations in food compositions is somewhat limited by the pronounced resistance of such substances to digestion. Since the esterified alkoxylated polyols are hydrolyzed and absorbed to only a very limited degree, they tend to retain their oil-like physical characteristics after ingestion. Consumption of large amounts of the fat substitutes can result in short bowel transit times and undesired laxative effects.
To enhance the acceptability of fat substitutes of this type, modified esterified alkoxylated glycerins have been developed which are somewhat less resistant to enzymatic hydrolysis than previously known esterified alkoxylated glycerins and yet still have significantly reduced calorie availability as compared to a conventional fully digestible triglyceride lipid. These modified esterified alkoxylated glycerins have one or two fatty acid ester groups attached directly to the carbons of the glyceryl radical. These directly attached ester groups are fairly readily hydrolyzed upon ingestion, rendering the compounds less fat-like in character owing to the loss of one or more long-chain fatty acid groups.
The ester groups in the esterified alkoxylated glycerin which are attached to the glyceryl radical through polyoxypropylene segments are resistant towards enzymatic hydrolysis. The structures of two preferred embodiments of such modified esterified alkoxylated polyols are as follows: ##STR2## wherein R is a long-chain hydrocarbon radical derived from fatty acid.
The synthesis of esterified alkoxylated mono- or diglycerides of this type is not straightforward. Esterified propoxylated glycerin may be prepared by reacting glycerin with propylene oxide in the presence of a basic alkali metal catalyst to form a propoxylated glycerin. The propoxylated glycerin is then esterified with a fatty acid compound such as a free fatty acid, fatty acid ester, or fatty acid halide. Using this synthetic approach, however, it is not possible to have an ester group attached directly to the glyceryl residue since the propylene oxide tends to add in a random fashion to all three hydroxyl groups of the starting glycerin: ##STR3##
A possible alternative method of preparation of an esterified alkoxylated monoglyceride would be to propoxylate a fatty acid monoglyceride and then esterify the secondary hydroxyl groups of the propoxylated monoglyceride: ##STR4## However, when this procedure is attempted, the product obtained is similar to the esterified propoxylated glycerin known in the prior art wherein oxypropylene units are present between the glyceryl radical and all three of the ester groups. Apparently, transesterification readily takes place under the reaction conditions necessary to achieve propoxylation of the fatty acid monoglyceride: ##STR5##
Thus, it is apparent there is a great need for processes whereby an esterified alkoxylated polyol having at least one ester group attached directly to the polyol residue may be readily prepared. The use of a ketal or acetal protective group to "mask" or "block" one or more hydroxyl groups is a commonly employed synthetic method in organic chemistry. Since a ketal or acetal group is normally nonreactive under basic conditions, other functional groups in an organic compound having such a protective group may be readily transformed using a basic reagent while not disturbing the ketal or acetal. The protective group may then be removed by acid-catalyzed hydrolysis and the free hydroxyl group(s) thereby generated subsequently further reacted in any desired manner.
This synthetic approach was employed in U.S. Pat. No. 4,581,470 to prepare 1,2- and 1,3-extender polyols useful in polyurethane formulations. The reference indicates, for example, that a starter such as glycerin may be reacted with a blocking agent such as acetone to form a blocked triol starter derivative (e.g., isopropylidene glycerin, which may also be referred to as 2,2-dimethyl-1,3-dioxolane-4-methanol). The remaining free hydroxyl group is subsequently reacted with an alkylene oxide such as propylene oxide. The alkoxylated product thereby obtained is then treated with an aqueous acidic solution to hydrolyze the ketal functional group and unblock the protected hydroxyl groups. It may be readily seen that the deprotected alkoxylated glycerin thereby obtained could be esterified with a fatty acid or fatty acid derivative to yield a modified esterified alkoxylated glycerin fat substitute of the type described hereinabove. This reaction scheme may be illustrated as follows: ##STR6##
However, a multi-step process of this type will not be commercially attractive unless the individual steps involved can be simplified and streamlined as much as possible. The hydrolysis step, wherein protected alkoxylated polyol 3 is transformed to deprotected alkoxylated polyol 4, is particularly problematic since it involves an acid and substantial amounts of water, both of which must eventually be separated from the deprotected alkoxylated polyol or any subsequent ultimate product. Alkoxylated intermediates such as 3 or 4, by analogy to conventional polyether polyols such as polypropylene glycol or polyethylene glycol, would be expected to either have high water solubilities or a tendency to form emulsions in water. Additionally, as n is increased, the viscosity of such intermediates tends to increase thereby requiring the use of organic solvents. Subsequent complete removal of organic solvent will be necessary if the final esterified alkoxylated polyol is to be safely used as a fat substitute in food compositions.
In U.S. Pat. No. 4,581,470, the hydrolysis step is accomplished by treating the ketal-containing alkoxylated intermediate with aqueous sulfuric acid at an elevated temperature (110.degree. C.) while simultaneously removing a portion of the water by distillation. The hydrolyzed product was then neutralized with aqueous potassium hydroxide and then stripped under high vacuum to remove the remaining water. The crude product required further treatment with magnesium silicate to remove the salts formed during neutralization. It may thus be seen that hydrolysis required at least four discrete steps. The overall process was consequently fairly lengthy and tedious since it involved removal of all the water present by distillation as well as a prolonged treatment to remove salts.
From the foregoing discussion, there is clearly a need for an improved and simplified process wherein a ketal protective group may be readily and conveniently removed from a protected alkoxylated polyol.